For centuries, all the world was convinced that the earth stood still because we felt nothing of its movement, it was our reference system. As late as the sixteenth century, Martin Luther, speaking at a table, said: "As the scriptures show, Joshua left the sun standing and not the earth." And his contemporary the great astronomer Tycho Brahe stated that the earth is lazy and inert and not suitable for movement. Through Nicolaus Copernicus the thesis of the resting earth was no longer tenable. Now, of course, one assumed a resting universe.
But Albert Einstein transferred his principle of relative motion to the whole cosmos. The absolutely resting space in which everything took place no longer existed. Now suddenly everything was relative. For Einstein there was no world ether, for him the Michelson experiment was comparable to a ball game in a moving train.
It seems completely natural to us that the ball moves in the same way as if you were playing in a meadow, provided that the train is travelling straight ahead at the same speed. If we replace the moving train with a coordinate system that moves straight through space at a constant speed, then according to Albert Einstein all the natural laws known to us in this system apply just as if the coordinate system were stationary.
The same of course applies to any coordinate system that has a different direction and a different speed. Each system is a world in its own right. All systems are equal to each other. The principle sounds simple and convincing.
The absolutely resting space that always remained the same and in which all natural processes took place, whose points could be measured in length, width and height, which was filled with that mysterious light medium, the ether, this space no longer existed.
If Einstein's principle of relativity was correct, i.e. if the same laws of nature prevailed in every system, then light could not form an exception, then one would have to measure the same speed of light in every system. So Einstein concluded that the speed of light in a vacuum must always the same, no matter how fast the light source moves and there must no higher speed in the universe than that of light.
The postulate of the speed of light as a universal natural constant brought with it a problem. Let us simplify the speed of light by replacing it with the average speed of our game ball. As long as we travel in the coordinate system in which the ball game takes place, everything is in order. In accordance with Einstein's principle, we always observe the same ball speed. But now we change our observation point. A train in which the example runs past us. From our current point of view, the ball is so fast that our eye can no longer follow it, because its speed adds up with that of the train.
But if we move our game to an airplane that flies at supersonic speed, the observer on Earth would have to determine ball speeds that are completely impossible for a child's ball. It flies faster than we do. But since the speed of light must always be the same as the speed of our ball, Albert Einstein simply questioned the measure of time. The faster a coordinate system moves, the slower time has to run in this system, if you look at it from the outside. Every system has its own inherent measure of time. Thus the constant speed of light is maintained. For a traveler in this system nothing changes. He takes everything in the usual way.
